This invention relates to composite energy control sheets and to window units incorporating such sheets.
For at least the past several decades many people have striven to develop a window cover or shade which would allow sunlight's visible wave lengths (0.4-0.7 micrometer) to pass through a glazed window into a room but which would simultaneously reduce glare and exclude sunlight's heat-generating near infrared wave lengths (0.7-2.5 micrometers). Lion, U.S. Pat. No. 2,774,021, for example, discloses a window shade in which a transparent or translucent self-supporting cellulosic foil is coated with a transparent-reflective layer of metal on the side adjacent the window, a protective layer of varnish or the like optionally being applied over the metal layer to reduce corrosion or mechanical damage. Subsequent refinements of Lion's window unit (i.e., windowpane plus shade) have included inner storm windows, where the solar control sheet is sealed or attached to the window frame to provide a dead air space between the glass and the shade.
Antonson et al, U.S. Pat. No. 3,290,203 describes and claims a window unit in which a transparent foil (especially a polyester foil) is provided with a transparent-reflective metal layer on one face, the metal layer being covered by a transparent protective layer which in turn in adhered to the inside of a conventional windowpane. This type of window unit, which is simple, compact and convenient, has been found more effective than the Lion window unit in blocking the entry of solar-generated near infra-red energy into a room. The transparent protective layer may be either a coating or, if desired, a second polyester foil adhered in place, as is shown in, e.g., Windorf, U.S. Pat. No. 3,775,226; in the latter case, either of the two polyester foils may be adhered to the windowpane. The adhesive which bonds the solar control sheet to the windowpane may be water-soluble (cf. the aforementioned Antonson et al patent), pressure-sensitive, pressure-sensitive but water-activatable (c.f. Theissen, U.S. Pat. 3,681,179), or of a "cling" nature (cf. Burger, U.S. Pat. 4,095,013).
As will be apparent from the foregoing discussion, work in the solar energy control sheet art has been almost exclusively concerned with keeping sunlight's heat and glare from affecting the comfort of those inside a room, hence, energy control sheets have been most widely used in those geographical areas where the outside temperature rarely falls below 0.degree. C. Studies have shown, however, that windows not only contribute heavily to high air conditioning energy usage in the summer but also contribute significantly to high heating costs in winter. The thermal conductance (or "U" value*) of a single glazed window typically exceeds 5 kcal/.degree.C. hr. m.sup.2, whereas a well-insulated wall has a U value less than 0.5 and a well-insulated ceiling has a U value less than 0.2. Thus, heat can be lost through a conventionally glazed window at a rate over an order of magnitude greater than through insulated walls or ceilings. In cold regions, attempts to make the occupants of a room comfortable in winter have usually involved adding external storm windows (which is not always feasible) and drawing opaque drapes across the face of the window, thereby blocking any view of the outside. FNT *The insulative (or "R" value) is the reciprocal of a "U" value; e.g., a U value of 0.1 is equivalent to an R value of 10. In English units, U is expressed as BTU/.degree.F. hr. ft.sup.2. In metric units, U is expressed as kcal/.degree.C. hr. m.sup.2.
Prior to the present invention, those persons in cold countries who occupied rooms where the windows were protected with solar control sheets (especially where such sheets were adhered to the inner surface of a windowpane) often felt cold in winter for at least two reasons. Solar-originating near infra-red energy (0.7-2.5 micrometers) was reflected back outside; additionally, heat inside the room was transferred to the windowpane by radiation and convection and lost to the outside. Nearly half of such loss was caused by internal infra-red energy (wave length range of about 4-40 micrometers) radiating from the skin of room occupants, as well as from the outside of objects in the room, to the solar control sheet, where it was absorbed by the foil adjacent the room, transmitted by conduction to the metalized layer, further conducted to the windowpane and then radiated to the outside. Although there has been a desire to maintain the visual advantages of solar energy control sheets while improving their poor insulative properties, no means for achieving such an objective has previously been provided. Thus, for example, it has been suggested* that the conventional infra-red absorbing polyester foil could be replaced with polyethylene (which is relatively transparent to infra-red rays), but it has been found impractical to apply a metal vapor coating to polyethylene for use in a solar energy control sheet, one or more of such properties as handling, adhesion, reflectivity and optical clarity being unsatisfactory. FNT *See, e.g., American Institute of Physics Conference Proceedings, No. 25, Efficient Use of Energy, Part III, p. 292, New York, NY, 1975